Python system version vs python Blender internal version
Blender comes bundled with it's own Python. It doesn't have anything to do with your system Python and you shouldn't try to mix the two, this will only give you headaches. Stay with whatever Python version Blender provides for your Blender scripting.

If you need additional libraries for Blender's python, you can install them in Blender's Python directory (in the directory you installed Blender in, navigate to blenderversion/python/lib to find the standard python library path), or install the libaries somewhere else and adjust sys.path in your python scripts before you import them.

There are reasons why Blender does not use the System Python installation. The first one is that no Python might be installed, but much more importantly, Blender's python API is developed against a specific Python version - the one bundled with Blender - and Blender can't guarantee that it will work with any other version of Python. If you try to trick Blender into using your System Python, you might encounter strange bugs nobody else has seen. This way lies madness.

Note that even pure Python projects often use virtual environments to isolate themselves from the System Python installation and provide a known environment.